The weakening and disappearance of the magnetic field on Mars was due primarily to the planet's size. A planetary magnetic field is created by a dynamo effect of molten metal sloshing around near to the core of the planet. This molten metal creates a current, and a moving current generates a magnetic field. However, because Mars is small, it's core cooled quickly (much quicker than Earth's for example) and so its core solidified. The solid core no longer creates a current and so the its dynamo stops. With its dynamo shut down, it no longer had a magnetic field to deflect charged particles in the solar wind from impacting and stripping its atmosphere. It's small size also contributed to this latter effect as its gravity is weaker meaning it was easier for the solar wind to erode the Martian atmosphere.

The link you provided [polarjournal.ch] details a time when the Earth's magnetic field weakened and briefly reversed, around 42,000 years ago. This period in time is known as the Laschamps excursion (or Laschamps Event) or, now known as the Adam's event (named after the author Douglas Adams who wrote The Hitchhiker's Guide to the Galaxy, in which the number 42 is jokingly referred to as the answer to the question "what is the meaning of Life, the universe, and everything?"). It is important to note that this event, while quick and dramatic in geological time, only lasted 1300 years, before the field returned to normal. It is also important to note that this magnetic field excursion is different to a change in the Earth's axial tilt, or the hypothetical Earth Crust Displacement. A change in axial tilt is when the Earth's spin axis, with respect to the ecliptic plane (i.e. the plane of its orbit about teh Sun) changes. Earth Crustal Displacement is when the entire crust of the earth moves as a whole, sliding over mantel below. Compare this to a geomagnetic excursion, where the direction of the magnetic pole moves or reverses magnetic polarity, without any movement of the rest of the planet.

For those interested, we do not know the exact date of the Laschamps event, only that it was around 42,000 years ago. It was first observed in the magnetic record in lava flows in Laschamps, France, but it has now been observed in precise detail in sub-fossil Kauri trees from New Zealand.

Kauri are long lived trees that are native New Zealand. Being long lived (they can live up to around 2000 years), they are considered ideal for long term climatological study, as well as dendrochronology. We only have an absolute Kauri tree-tree ring chronology stretching back from AD 2002 to around 4500 years ago. Before this, there are a number of floating chronologies ranging between hundreds of years to 1000-2000 year long, spanning the last 60,000 years (apart from the period 13,000-27,000 years ago; for some reason this period has yielded very little wood). Therefore to get a date for these floating chronologies they must be radiocarbon dated.

The Laschamps Event appears Kauri trees as a change in radiocarbon concentration, particularly as a sharp change in tree a Kauri tree that lived for 1700 years across the Leschamps event (the rate of radiocarbon production is related to the strength of the Geomagnetic field) . However, to get a better feel of an absolute calendar date, the Kauri radiocarbon dates were spliced into a radiocarbon sequence from a Hulu Cave Speleothem (a stalagmite). Speleothems have their own growth layers, or laminae, (kind of like tree rings), and so these layers can be "counted" (and so can be considered absolute). The Hulu Cave Speleothem was used to extend the radiocarbon calibration curve back to around 54,000 years ago (almost to the very limit of radiocarbon dating) by comparing the dates of Uranium-Thorium dating of calcium carbonate layers to radiocarbon dates from the same calcium carbonate layers of the speleothem. By comparing the radiocarbon data of the 1700 year old Kauri to the Hulu cave radiocarbon data, "absolute dates" can,in theory, be given (whether the laminae of some speleothems are absolutely dated is debatable).

The full paper describing the above research can be read here [www.researchgate.net].

Jonny

The path to good scholarship is paved with imagined patterns. - David M Raup